and crashed into a huge advertising sign along a street just 300 yards from the airport. The airframe burned. 12 March The Lockheed AH-56 Cheyenne test programme suffers a setback when the rotor on prototype #3, 66–8828, hits the fuselage and kills the pilot, David A. Beil. The accident occurs on a test flight where the pilot was to manipulate the controls to excite 0.5P oscillations (or half-P hop) in the rotor. 0.5P is a vibration that happens once per two main rotor revolutions, where P is the rotor rotational speed. The accident investigation noted that safety mechanisms on the controls had apparently been disabled for the flight. The investigation concluded that the pilot-induced oscillations had set up a resonant vibration that exceeded the rotor system's ability to compensate. After the investigation, the rotor and control systems were modified to prevent the same problem from occurring again. The rotor blades and control system were stiffened, the mass of the gyro was increased, and the geometry of the rotor was adjusted. 14 March Lockheed XV-4B Hummingbird, 62-5404, on a conventional test flight out of Dobbins AFB, Georgia, suddenly enters rapid roll while climbing through 8,000 feet (2438 m), pilot Harlan J. "Hal" Quamme, unable to recover, ejects, suffering minor injuries. One civilian on ground receives minor injuries as well. Aircraft impacts ~20 miles from Dobbins AFB. 7 April A Mitsubishi-assembled, from Sikorsky components, Sikorsky HSS-1N Seabat, 8567, c/n 58-1528, of the Japanese Maritime Self-Defense Force, crashes, the sole loss of the type during Japanese service. 11 April Lockheed SR-71A, 61-7954, Article 2005, crashes on runway during take off from Edwards Air Force Base, California. Pilot Lt. Col. Bill Skliar and RSO Maj. Noel Warner escape without injury. 15 April North Korean Mikoyan-Gurevich MiG-17s shoot down a Navy Lockheed EC-121M Warning Star, BuNo 135749, 'PR 21', c/n 4316, of VQ-1, call sign "Deep Sea 129", over the Sea of Japan, killing all 31 aboard. See EC-121 shootdown incident. KWF are pilot L/Cdr. James H. Overstreet; Lt. John N. Dzema; Lt. Dennis B. Gleason; Lt. Peter P. Perrottet; Lt. John H. Singer; Lt. Robert F. Taylor; Lt.JG Joseph R. Ribar; Lt.JG Robert J. Sykora; Louis F. Balderman, ADR2; Stephen C. Chartier, AT1; Bernie J. Colgin, AT1; Ballard F. Connors, Jr, ADR1; Gary R. DuCharme, CT3; Gene K. Graham, ATN3; LaVerne A. Greiner, AEC; Dennis J. Horrigan, ATR2; Richard H. Kincaid, ATN2; Marshall H.